Is there any way to reset all passwords previously created when the secondary password file is enabled for the first time to ensure higher-level protection ?.
You can choose to expire all user accounts with a loop using the
passwd command, forcing them to change their password on next login. Avoid this on system accounts. Eg, user accounts on Linux Debian based systems start at uid 1000. Most system accounts have a disabled password anyway.
Immediately expire an account's password. This in effect can force a user to change his/her password at the user's next login.
There are other related options to take care about (like
-i, --inactive which might later disable such accounts).
Use a loop iterating on users (eg using contents of
/etc/passwd or more generic using
getent passwd [someuser]